Activated scaling of classical and quantum spin glasses

A model for thermally activated dynamics in disordered systems shows that the linear and nonlinear susceptibility follows a generic exponential form with a "critical rounding," chi(1) proportional to chi(3) proportional to [T ln(t/tau(0)')/K](gamma/b phi) exp - [Tt(g)(phi b)ln(t/tau(0)'/K)](nu/b) (T=temperature, t=time, K=barrier constant, t(g) = 1 - T(SG)/T, and T(SG) = transition temperature; gamma>0 for chi(3) and <0 for chi(1)). This model, also valid in the presence of resonant tunneling states at energies K(0) < K [provided that K is replaced by K(0)+2T ln (1/Gamma(0)), where Gamma(0)(2) proportional, variant tunnel splitting of a spin S=1], is potentially applicable to a wide variety of systems opening the way for the study of thermally activated quantum phase transitions. The famous spin-glass system LiHo(x)Y(1-x) seems to follow this model.     

Effects of a parallel magnetic field on the metal-insulator transition in a dilute two-dimensional electron system

The temperature dependence of conductivity sigma(T) of a two-dimensional electron system in silicon has been studied in parallel magnetic fields B. At B = 0, the system displays a metal-insulator transition at a critical electron density n(c)(0), and dsigma/dT>0 in the metallic phase. At low fields ( B < or approximately equal to 2 T), n(c) increases as n(c)(B)-n(c)(0) proportional, variant Bbeta ( beta approximately 1), and the zero-temperature conductivity scales as sigma(n(s),B,T = 0)/sigma(n(s),0,0) = f(B(beta)/delta(n)), where delta(n) = [n(s)-n(c)(0)]/n(c)(0) and n(s) is electron density, as expected for a quantum phase transition. The metallic phase persists in fields of up to 18 T, consistent with the saturation of n(c) at high fields.     

Crossover between ordinary and normal transitions in two dimensional critical Ising films

We investigate two dimensional critical Ising films of width L with surface fields H(1)=H(L) in the crossover between ordinary (H(1)=0) and normal (H(1)=infinity) transitions. Using exact transfer-matrix diagonalization and density matrix renormalization-group (DMRG) methods, we calculate magnetization profiles m(z), the excess magnetization Gamma, and the analog of the solvation force f(solv) as functions of H1 for several L. Scaling functions of the above quantities deviate substantially from their asymptotic forms at fixed points for a broad region of the scaling variable LH21 approximately L/l(1), where l(1) is the length induced by the surface field H1. The scaling function for /f(solv)/ has a deep minimum near LH(2)(1)=1, which is about one order of magnitude smaller than its value at both fixed points (the "Casimir" amplitude). For weak H1 (l(1)>L) the magnetization profile has a maximum at the center of the film, and f(solv) decays much faster than L-2. For stronger H1 (1l(1) the solvation force decays according to the universal power law f(solv) approximately L(-2). The results of the approximate DMRG method show remarkable agreement with the exact ones.     

Bose-Einstein condensation of magnons in Cs2CuCl4

We report on results of specific heat measurements on single crystals of the frustrated quasi-2D spin-1/2 antiferromagnet Cs2CuCl4 (T(N)=0.595 K) in external magnetic fields B<12 T and for temperatures T>30 mK. Decreasing B from high fields leads to the closure of the field-induced gap in the magnon spectrum at a critical field Bc approximately = 8.51 T and a magnetic phase transition is clearly seen below Bc. In the vicinity of Bc, the phase transition boundary is well described by the power law Tc(B) proportional, variant (Bc-B)(1/phi), with the measured critical exponent phi approximately =1.5. These findings are interpreted as a Bose-Einstein condensation of magnons.     

Dynamical response of quantum spin-glass models at t = 0

We study the behavior of two archetypal quantum spin glasses at T = 0 by exact diagonalization techniques: the random Ising model in a transverse field and the random Heisenberg model. The behavior of the dynamical spin response is obtained in the spin-glass ordered phase. In both models it is gapless and has the general form chi(")(omega) = qdelta(omega)+chi(")(reg)(omega), with chi(")(reg)(omega) approximately omega for the Ising and chi(")(reg)(omega) approximately const for the Heisenberg, at low frequencies. The method provides new insight to the physical nature of the low-lying excitations.     

Enhancement of the quantum-liquid phase by increased resistivity in thick a-MoxSi1-x films

Effects of normal-state resistivity rho(n) on the vortex phase diagram at low temperature T have been studied based on dc and ac complex resistivities for thick amorphous MoxSi(1-x) films. It is commonly observed irrespective of rho(n) that, in the limit T=0, the vortex-glass-transition line B(g)(T) is independent of T and extrapolates to a field below the T=0 upper critical field B(c2)(0), indicative of the quantum-vortex-liquid (QVL) phase in the regime B(g)(0)相似文献   

Semileptonic decays of d mesons in three-flavor lattice QCD

We present the first three-flavor lattice QCD calculations for D-->pilnu and D-->Klnu semileptonic decays. Simulations are carried out using ensembles of unquenched gauge fields generated by the MILC Collaboration. With an improved staggered action for light quarks, we are able to simulate at light quark masses down to 1/8 of the strange mass. Consequently, the systematic error from the chiral extrapolation is much smaller than in previous calculations with Wilson-type light quarks. Our results for the form factors at q(2)=0 are f(D-->pi)(+)(0)=0.64(3)(6) and f(D-->K)(+)(0)=0.73(3)(7), where the first error is statistical and the second is systematic, added in quadrature. Combining our results with experimental branching ratios, we obtain the Cabibbo-Kobayashi-Maskawa matrix elements |V(cd)|=0.239(10)(24)(20) and |V(cs)|=0.969(39)(94)(24), where the last errors are from experimental uncertainties.     

Dephasing relaxation of the electron spin echo of the vibronic Cu(H(2)O)(6) complexes in Tutton salt crystals at low temperatures.

Two-pulse electron spin echo (ESE) measurements of the phase relaxation (phase memory time T(M)) were performed in a series of Tutton salt crystals M(I)(2)M(II)(SO(4))(2).6X(2)O (M(I)=NH(4), K, Cs; M(II)=Zn, Mg; X=H, D) weakly doped with Cu(2+) ions (c approximately equal to 10(18) ions/cm(3)) in temperature range 4-60 K where ESE signals were detectable. The ESE decay was strongly modulated with proton (or deuteron) frequencies and described by the decay function V(2tau)=V(0)exp(-btau-mtau(2)) with the mtau(2) term being temperature independent and negligible above 20 K. Various mechanisms leading to the tau- or tau(2)-type ESE decay are reviewed. The m and b coefficients for nuclear spectral diffusion (NSD), electron spectral diffusion (SD), and instantaneous diffusion (ID) were calculated in terms of existing theories and the resulting rigid lattice T(0)(M) times were found to be close one to another within the crystal family with average values: 17.5 micros (NSD protons), 200 micros (NSD deuterons), 8 micros (SD), and 5 micros (ID). The ID dominates but the calculated effective T(M)(0) is longer than the experimental T(M)(0)=2 micros. This is due to a nonuniform distribution of the Cu(2+) ions with a various degree of the disorder in the studied crystals. The acceleration of the dephasing rate 1/T(M) with temperature is due to the mechanisms producing exp(-btau) decay. They are reviewed and two of them were found to be operative in Tutton salt crystals: (a) Excitations to the vibronic levels of energy Delta leading to the temperature dependence 1/T(M)=B exp(-Delta/kT), with the vibronic levels produced by strong Jahn-Teller effect, and (b) spin-lattice relaxation processes being effective above 50 K. Based on the Delta values being on the order of 100 cm(-1), the scheme of vibronic levels in the Tutton salts is presented, and the independence of the Delta on temperature proves that the adiabatic potential surface shape of Jahn-Teller active Cu(H(2)O)(6) complexes is not affected by temperature below 65 K.     

Experimental and Theoretical Study of the Electronic States and Spectra of TeH and TeLi

Gas-phase emission spectra of the hitherto unknown free radical TeLi have been measured in the NIR range with a Fourier-transform spectrometer. The emissions were observed from a fast flow system in which tellurium vapor in argon carrier gas was passed through a microwave discharge and mixed with lithium vapor in an observation tube. Two systems of blue-degraded bands were measured at high spectral resolution in the ranges 8000-9000 and 5700-6700 cm(-1) and vibrational and rotational analyses were performed. In order to aid in the analysis of the experimental data, a series of relativistic configuration interaction calculations has been carried out to obtain potential curves for the low-lying states of TeLi and the isovalent TeH and also electric dipole transition moments connecting them. As in the TeH system, the ground state of TeLi is found to be X(2)Pi(i), but with a remarkably smaller spin-orbit splitting. The TeLi calculations indicate a strongly bound A(2)Sigma(+) state, while in TeH the analogous state is computed to lie significantly higher at approximately 32 000 cm(-1), and it is strongly predissociated. Based on the theoretical analysis, the observed TeLi band systems are assigned to the transitions A(2)Sigma(+)(A1/2)-->X(1)(2)Pi(3/2)(X(1)3/2) and A(2)Sigma(+)(A1/2)-->X(2)(2)Pi(1/2)(X(2)1/2). Analysis of the spectra has yielded the molecular constants (in cm(-1)) X(1)(2)Pi(3/2):omega(e)=457.49(3), omega(e)x(e)=2.482(9), B(0)=0.408908(8); X(2)(2)Pi(1/2): T(e)=2353.44(3), omega(e)=456.28(4), omega(e)x(e)=2.635(8), B(0)=0.414954(8), p(0)=1.00637(4); A(2)Sigma(+): T(e)=8574.64(2), omega(e)=437.81(3), omega(e)x(e)=2.581(8), B(0)=0.423903(8), p(0)=-0.19915(2), where the numbers in parentheses are the standard deviations of the parameters. Comparison of the isovalent TeLi and TeH systems emphasizes that the difference in bonding character (ionic in TeLi vs covalent in TeH) is responsible for qualitative differences in the electronic spectra of these two molecules. Copyright 2001 Academic Press.     

Ferromagnetic quantum critical fluctuations in YbRh2(Si0.95Ge0.05)2

The bulk magnetic susceptibility chi(T,B) of YbRh(2)(Si(0.95)Ge(0.05))(2) has been investigated close to the field-induced quantum critical point at B(c) = 0.027 T. For B < or= 0.05 T a Curie-Weiss law with a negative Weiss temperature is observed at temperatures below 0.3 K. Outside this region, the susceptibility indicates ferromagnetic quantum critical fluctuations, chi(T) proportional, variantT-0.6 above 0.3 K. At low temperatures the Pauli susceptibility follows chi(0) proportional, variant(B-B(c))(-0.6) and scales with the coefficient of the T(2) term in the electrical resistivity. The Sommerfeld-Wilson ratio is highly enhanced and increases up to 30 close to the critical field.     

Hydration water dynamics in biopolymers from NMR relaxation in the rotating frame

Assuming dipole-dipole interaction as the dominant relaxation mechanism of protons of water molecules adsorbed onto macromolecule (biopolymer) surfaces we have been able to model the dependences of relaxation rates on temperature and frequency. For adsorbed water molecules the correlation times are of the order of 10(-5)s, for which the dispersion region of spin-lattice relaxation rates in the rotating frame R(1)(ρ)=1/T(1)(ρ) appears over a range of easily accessible B(1) values. Measurements of T(1)(ρ) at constant temperature and different B(1) values then give the "dispersion profiles" for biopolymers. Fitting a theoretical relaxation model to these profiles allows for the estimation of correlation times. This way of obtaining the correlation time is easier and faster than approaches involving measurements of the temperature dependence of R(1)=1/T(1). The T(1)(ρ) dispersion approach, as a tool for molecular dynamics study, has been demonstrated for several hydrated biopolymer systems including crystalline cellulose, starch of different origins (potato, corn, oat, wheat), paper (modern, old) and lyophilized proteins (albumin, lysozyme).     

Anti-de sitter/conformal-field-theory calculation of screening in a hot wind

One of the challenges in relating experimental measurements of the suppression in the number of J/psi mesons produced in heavy ion collisions to lattice QCD calculations is that whereas the lattice calculations treat J/psi mesons at rest, in a heavy ion collision a cc[over ] pair can have a significant velocity with respect to the hot fluid produced in the collision. The putative J/psi finds itself in a hot wind. We present the first rigorous nonperturbative calculation of the consequences of a wind velocity v on the screening length L(s) for a heavy quark-antiquark pair in hot N=4 supersymmetric QCD. We find L(s)(v,T)=f(v)[1-v(2)](1/4)/piT with f(v) only mildly dependent on v and the wind direction. This L(s)(v,T) approximately L(s)(0,T)/sqrt[gamma] velocity scaling, if realized in QCD, provides a significant additional source of J/psi suppression at transverse momenta which are high but within experimental reach.     

Evidence for a nodeless gap from the superfluid density of optimally doped Pr(1.855)Ce(0.145)CuO(4-y) films

We present measurements of the ab-plane magnetic penetration depth, lambda(T), in five optimally doped Pr(1.855)Ce(0.145)CuO(4-y) films for 1.6 K< or =T < or =T(c) approximately 24 K. Low resistivities, high superfluid densities n(s)(T) proportional, variant lambda(-2)(T), high T(c)'s, and small transition widths are reproducible and indicative of excellent film quality. For all five films, lambda(-2)(T)/lambda(-2)(0) at low T is well fitted by an exponential temperature dependence with a gap, Delta(min), of 0.85k(B)T(c). This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.     

Exact phase diagrams for an Ising model on a two-layer Bethe lattice

Using an iteration technique, we obtain exact expressions for the free energy and the magnetization of an Ising model on a two-layer Bethe lattice with intralayer coupling constants J1 and J2 for the first and the second layer, respectively, and interlayer coupling constant J3 between the two layers; the Ising spins also couple with external magnetic fields, which are different in the two layers. We obtain exact phase diagrams for the system and find that when /J3/-->0, DeltaT(c) identical with[T(c)(J3)-T(c)(0)]/T(c)(0) approximately [J3]/J(1)/(1/psi), where T(c)(J3) is the phase-transition temperature for the system with interlayer coupling constant J3 and the shift exponent psi is 1 for J(1)=J(2) and is 0.5 for J1 not equal to J2. Such results are consistent with predictions of a scaling theory. We also derive equations for DeltaT(c) when /J3/ approaches infinity.     

B-meson decay constant from unquenched lattice QCD

We present determinations of the -meson decay constant f(B) and f(B)(s)/f(B) using the MILC Collaboration unquenched gauge configurations, which include three flavors of light sea quarks. The mass of one of the sea quarks is kept around the strange quark mass, and we explore a range in masses for the two lighter sea quarks down to m(s)/8. The heavy quark is simulated using nonrelativistic QCD, and both the valence and sea light quarks are represented by the highly improved (AsqTad) staggered quark action. The good chiral properties of the latter action allow for a more accurate chiral extrapolation to physical up and down quarks than has been possible in the past. We find f(B)=216(9)(19)(4)(6) MeV and f(B)(s)/f(B)=1.20(3)(1).     

Diffusion and scaling in escapes from two-degrees-of-freedom Hamiltonian systems

This paper summarizes an investigation of the statistical properties of orbits escaping from three different two-degrees-of-freedom Hamiltonian systems which exhibit global stochasticity. Each time-independent H=H(0)+ varepsilon H('), with H(0) an integrable Hamiltonian and varepsilon H(') a nonintegrable correction, not necessarily small. Despite possessing very different symmetries, ensembles of orbits in all three potentials exhibit similar behavior. For varepsilon below a critical varepsilon (0), escapes are impossible energetically. For somewhat higher values, escape is allowed energetically but still many orbits never escape. The escape probability P computed for an arbitrary orbit ensemble decays toward zero exponentially. At or near a critical value varepsilon (1)> varepsilon (0) there is a rather abrupt qualitative change in behavior. Above varepsilon (1), P typically exhibits (1) an initial rapid evolution toward a nonzero P(0)( varepsilon ), the value of which is independent of the detailed choice of initial conditions, followed by (2) a much slower subsequent decay toward zero which, in at least one case, is well fit by a power law P(t) proportional, variant t(-&mgr;), with &mgr; approximately 0.35-0.40. In all three cases, P(0) and the time T required to converge toward P(0) scale as powers of varepsilon - varepsilon (1), i.e., P(0) proportional, variant ( varepsilon - varepsilon (1))(alpha) and T proportional, variant ( varepsilon - varepsilon (1))(beta), and T also scales in the linear size r of the region sampled for initial conditions, i.e., T proportional, variant r(-delta). To within statistical uncertainties, the best fit values of the critical exponents alpha, beta, and delta appear to be the same for all three potentials, namely alpha approximately 0.5, beta approximately 0.4, and delta approximately 0.1, and satisfy alpha-beta-delta approximately 0. The transitional behavior observed near varepsilon (1) is attributed to the breakdown of some especially significant KAM tori or cantori. The power law behavior at late times is interpreted as reflecting intrinsic diffusion of chaotic orbits through cantori surrounding islands of regular orbits. (c) 1999 American Institute of Physics.     

New evidence for zero-temperature relaxation in a spin-polarized fermi liquid

Spin-echo experiments are reported for 3He-4He solutions under extremely high B/T conditions, B=14.75 T and T>or=1.73 mK. The 3He concentration x(3) was adjusted close to the value x(c) approximately 3.8% at which the spin-rotation parameter muM0 vanishes. In this way the transverse and longitudinal spin-diffusion coefficients D( perpendicular ),D( parallel ) were measured while keeping |muM(0)|<1. It is found that the temperature dependence of D( perpendicular ) deviates strongly from 1/T(2), with anisotropy temperature T(a)=4.26(+0.18)(-0.44) mK. This value is close to the theoretical prediction for dilute solutions and suggests that spin current relaxation remains finite as the temperature tends to zero.     

Measurement of the B0s lifetime in the exclusive decay channel B0s-->J/psiphi

Using the exclusive decay B0s-->J/psi(mu+mu-)phi(K+K-), we report the most precise single measurement of the B0s lifetime. The data sample corresponds to an integrated luminosity of approximately 220 pb(-1) collected with the D0 detector at the Fermilab Tevatron Collider in 2002-2004. We reconstruct 337 signal candidates, from which we extract the B0s lifetime, tau(B0s)=1.444(+0.098)(-0.090)(stat)+/-0.020(sys) ps. We also report a measurement for the lifetime of the B0 meson using the exclusive decay B0-->J/psi(mu+mu-)K*0(892)(K+pi-). We reconstruct 1370 signal candidates, obtaining tau(B0)=1.473(+0.052)(-0.050)(stat)+/-0.023(sys) ps, and the ratio of lifetimes, tau(B0s)/tau(B0)=0.980(+0.076)(-0.071)(stat)+/-0.003(sys).     

Dynamics of fluctuations below a stationary bifurcation to electroconvection in the planar nematic liquid crystal N4

We fitted C(k,tau,epsilon) proportional to exp([-sigma(k,epsilon)tau] to time-correlation functions C(k,tau,epsilon) of structure factors S(k,t,epsilon) of shadowgraph images of fluctuations below a supercritical bifurcation at V(0)=V(c) to electroconvection of a planar nematic liquid crystal in the presence of a voltage V=sqrt[2]V(0)cos((2pift) [k=(p,q) is the wave vector and epsilon identical with V(2)(0)/V(2)(c)-1]. There were stationary oblique (normal) rolls at small (large) f. Fits of a modified Swift-Hohenberg form to sigma(k,epsilon) gave f-dependent critical behavior for the minimum decay rates sigma(0)(epsilon) and the correlation lengths xi(p,q)(epsilon).     

Origin of the enhanced copper spin echo decay rate in the pseudogap regime of the multilayer high-T(c) cuprates

We report measurements of the anisotropy of the spin echo decay for the inner layer Cu site of the triple layer cuprate Hg(0.8)Re(0.2)Ba(2)Ca(2)Cu(3)O(8) (T(c)=126 K). The angular dependence of the second moment (T(-2)(2M) identical with ) deduced from the decay curves indicates that T(-2)(2M) for H0 parallel c is enhanced in the pseudogap regime below T(pg) approximately 170 K, as seen in bilayer systems. Comparison of T(-2)(2M) between H0 parallel c and H0 perpendicular c indicates that this enhancement is caused by electron spin correlations between the inner and the outer CuO2 layers. The results provide the answer to the long-standing controversy regarding the opposite T dependences of (T1T)(-1) and T(-2)(2G) (T(2G): Gaussian component) in the pseudogap regime of multilayer systems.